The present invention relates to optical transmission systems and more particularly to methods and systems for generating short pulse signals such as for high-speed transmission thereof.
Generation of ultra-short optical pulses is a necessity for high-speed transmission systems, such as those utilizing optical time division multiplexing. For a 160 Gbit/s OTDM signal, the pulse width should be smaller than 2 ps and have an extinction ratio of 30 dB or larger. Generating these ultra-short pulses in a simple, stable and high-quality manner, however, has proven challenging.
Electroabsorption modulators are currently thought to be one of the most practical return-to-zero (RZ) optical sources. Unfortunately, when an electroabsorption modulator is driven by a 10 GHz sinusoidal electrical signal, the pulse width at full width at half maximum (FWHM) is usually larger than approximately 20 ps. Compression is therefore necessary before the signal can be multiplexed into a higher bit rate.
One method of performing this compression is set forth in L. Boivin et al., xe2x80x9cA Supercontinuum Source based on an Electroabsorption-Modulated Laser for Long Distance DWDM Transmission,xe2x80x9d IEEE Photonics Technology Letters, Vol. 12, No. 12, December 2000, p.p. 1695-1697, the entirety of which is hereby incorporated by reference herein. The approach suggested by Boivin utilizes two nonlinear fiber stages to compress the signal pulses. Each stage includes an erbium doped fiber amplifier (EDFA), a dispersion-shifted fiber (DSF) length having a normal dispersion, a single-mode fiber (SMF) having an abnormal dispersion, an optical filter, a polarization controller and a polarizer. The first stage compresses a 21.5 ps pulse signal from an electroabsorption-modulated laser (EML) to 6 ps. The second stage is used to further reduce the pulse time from about 6 ps to about 2.7 ps. The optical filter is used to select a region of the spectrum having a linear chirp. Because the filter has its transparent wavelength far away from the center wavelength of the signal, the signal power is lost after this optical filter. Therefore, the amplifier is needed to amplify the signal before the second stage, and the amplified signal is then compressed in the second stage.
The approach of Boivin et al. provides short pulses having pulse widths as low as 2.7 ps, which are not appropriate for high-speed 160 Gbit/s or more OTDM transmissions. The approach also requires two compression stages, each including the aforementioned components. Also, because of the need to amplify the signal received from the first stage, the approach is somewhat power inefficient. Therefore, there remains a need for a simpler and more power efficient approach to generating short-pulse signals.
A short pulse generation system and method of generating a short pulse signal are provided. The short pulse generation system includes a fiber chain having an input for receiving an optical signal having a plurality of pulses. The fiber chain is nonlinear with respect to the optical signal. The fiber chain includes a first normal fiber segment having a relatively small chromatic parameter, a first abnormal fiber segment having a relatively large chromatic parameter coupled to an output of the first normal fiber segment, and a second abnormal fiber segment having a relatively small chromatic parameter coupled to an output of the first abnormal fiber segment. The system also includes a filter stage coupled to receive the optical signal from the fiber chain. The filter stage has a transparent wavelength selected to regenerate the optical signal. A short pulse signal is generated when the optical signal is coupled to the input. A simpler, stable and power efficient system and method for generating ultra short pulses is thereby provided. Also, by selecting a filter with two transmission channels, two short pulse signals can be produced, alleviating the need for an optical coupler for dividing the power of the optical signal and two optical filters for regenerating the divided optical signals, as would be necessary, for example, with the system of Boivin.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.